A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithographic apparatus, a background gas may be introduced, for example, a flushing gas or a background gas in a projection optics box in a lithographic apparatus operating in the extreme ultraviolet radiation range. The radiation, for example, EUV radiation, but also other radiation, for example, 157 nm, 193 nm and a particle beam, generated by the radiation source may be partially absorbed by the background gas. The radiation may also ionize the background gas, on-setting the creation of a plasma. The creation of free charges (electrons and ions) may lead to avalanche effects when the interaction of electrons and molecules leads to the creation of more free charges. This effect may be accelerated by electrical fields, which may be externally applied or radiation induced by the photo-electric effect. Under the influence of electric fields, the free charges in the plasma may reorganize themselves. Eventually, this may lead to quasi-neutrality in which there is no electrical field in the bulk of the plasma. Near a surface of the lithographic apparatus, a potential drop may arise. Accelerated ions in such potential drop may impact on surfaces in the lithographic apparatus, including optical elements such as mirrors and lenses, resulting in processes, such as etching and sputtering.
In particular, in a lithographic apparatus operating in the EUV range, electrical fields may be built up due to the strongly ionizing photons. These electrical fields lead to acceleration of ions and thus, to sputtering and etching inside the projection optics box. Solutions may include protecting the multilayer mirrors and other sensitive devices from etching/sputtering ions by the creation of electrical fields oriented away from the mirrors. However, creation of an electrical field away from mirrors may lead to an acceleration of ions from the mirrors to, for example, a lens structure. In that case, the etching and sputtering may take place at the lens structure instead, which is also undesirable, since products of the etching and sputtering processes may deposit on the mirrors, again leading to a decrease of the lifetime of expensive optical elements. Further, a transmission loss and/or a homogeneity loss may also occur. Some of the optical elements may be protected by a thin top layer whose removal is undesirable.